be my guest

Be my guest (a polite way of saying Yes you can.) — Пожалуйста, конечно, можно;

Example: "Can I borrow your phone to make a quick call?" — "Sure. Be my guest."

You can also say Help yourself

help yourself (to sth) (to take some of what you want, without asking permission — used especially when offering food to someone)

Example: Please help yourself to some cake.(LDOCE)